The surface of tumor cells often carry antigenic markers due to abnormal gene action or to the interaction with viruses which might make it amenable to attack by the host's immunologic defenses. We have examined the centromeric side of the murine MHC locus for evidence of genes which regulate either the expression of or function of MHC antigens in recognition of altered self. Fifteen new congenic lines have been developed for this purpose. We have studied the process of mutagenesis in an unusual strain of mice, POLY 2 less than 100/StAo (POLY 2). Two sublines of POLY 2 have been selected for high (line 2) and low (line 1) degrees of polydactyly. These lines show evidence of one-way and reciprocal rejection of skin grafts in inter-subline transplantation, the frequency of which has increased with each new generation. Pleiotropic effects (regarding coat color variants) have also been noted in the F1 (POLY 2 X AKR). The coat color variants do not fit one-gene, two-gene, or linked-gene models. The possibility is being examined that these are pleiotropic effects of retro viruses with unstable insertions. We are also continuing our examination of the fine specificity exhibited by cytotoxic T cell lines specific for SV40 transformants. A number of cytotoxic T cell lines have been generated in vitro. A series of cross-reactivity patterns have been observed with SV40 transformants of Kb mutant strains. We plan to clone and subclone cytotoxic T cell lines to examine further the specificity of these cells for SV40 transformants of Kb mutants with known amino substitutions. (LB)